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Abstract

We present a review of existing and potential next-generation far-infrared (20-60 μm) optical materials and devices. The far-infrared is currently one of the few remaining frontiers on the optical spectrum, a space underdeveloped and lacking in many of the optical and optoelectronic materials and devices taken for granted in other, more technologically mature wavelength ranges. The challenges associated with developing optical materials, structures, and devices at these wavelengths are in part a result of the strong phonon absorption in the Reststrahlen bands of III-V semiconductors that collectively span the far-infrared. More than just an underexplored spectral band, the far-IR may also be of potential importance for a range of sensing applications in astrochemistry, biology, and industrial and geological processes. Additionally, with a suitable far-IR optical infrastructure, it is conceivable that even more applications could emerge. In this review, we will present recent progress on far-infrared materials and phenomena such as phononic surface modes, engineered composite materials, and optoelectronic devices that have the potential to serve as the next generation of components in a far-infrared optical tool-kit.

Fig. 6 (a) Photocurrent spectrum of a double-step AlGaN FIR QWIP measured at 20K under an applied voltage bias of 0.8V (solid line), and Gaussian fit to the photocurrent data (dashed line). The grey band near the horizontal axis indicates the Reststrahlen band of GaN. The vertical arrow indicates the calculated intersubband transition energy (Reproduced with permission from Ref [75], Copyright 2012, AIP Publishing LLC. (b) Band structure, subband energy separations and wavefunctions of the active region of the proposed Al0.15Ga0.85N/GaN THz QCL. Two periods are shown with each period consisting of 3 GaN QWs and 3 Al0.15Ga0.85N barriers with layer thicknesses (Å): 30/40/30/25/20/25 (wells in bold and barrier in plain) under an electric bias of 70 kV/cm (Reproduced with permission from Ref [77], Copyright Elsevier).